This invention relates to deflection yokes for cathode ray tube display systems, and particularly to such yokes which introduce raster correcting non-linearity to the deflection causing magnetic fields.
The production of an accurate video image on a cathode ray tube (CRT) depends on an accurate scanning , raster on the CRT screen. CRT rasters are subject to many types of inaccuracies such as: pincushion, barrel, cupids bow (mustache or seagull), keystoning, and so on. Many of the geometric distortions can be eliminated by introducing the proper non-uniformities into the beam deflecting magnetic field shape or by driving the deflection yoke with non-linear sweep controlling signals. Nonlinear driving circuitry has met with only limited success however, since modifying the sweep signal generation circuitry to produce the needed non-uniform sweep signals adds to its expense and limits the specially designed driving circuitry to driving perhaps a single yoke and CRT combination. Such limitation of usage is significant with today's design methodologies where circuit modules are typically designed to be compatible with many types of related circuit arrangements to reduce design expense.
Another type of raster correction solution is to add fixed components such as permanent magnets or permeable pole pieces to the deflection yoke which constantly distort any applied magnetic field. The addition of such components is only an approximate solution and makes the yoke to which they are attached physically large and complex. Permeable pole pieces made of electrically conductive material are influenced by scan frequency, and permanent magnets must have a field strength selected for a given CRT anode voltage.
Accurate rasters can be produced by deflection yokes which are wound so that they produce required nonlinear deflection magnetic fields when they are driven by standard sweep signals. Nonlinear yokes are a preferred method of raster correction since they are compatible with standard sweep generation circuitry and they need only be somewhat more physically complex than linear yokes.
Yokes which produce non-uniform deflection fields do so by adding amounts of third, fifth or higher harmonic corrections to the deflection field. As the deflection angle, "squareness" and flatness of CRTs increases in keeping with consumer demand, the amount of required higher harmonic correction relative to the first harmonic increases substantially. In fact, the amount of such correction called for in modern CRTs frequently requires that the current flow direction in the deflection yoke turns used to create the higher harmonics must be opposite to the direction of current flow through the main coils in the same yoke quadrant. No acceptable yoke arrangement exists for producing such coil turns having a reversed current flow.
One known attempt to provide current flow opposite to that of a main winding is described in an article by B. B. Dasgupta in the 1987 S.I.D. Digest. With the arrangement described by Dasgupta an auxiliary coil which was energized in reverse of a main coil was added to provide horizontal turns at particular angular locations about the yoke. The addition of the auxiliary coil permitted turns of reversed current flow, but it added a full coil of conductors and reduced the inductance to resistance (L/R) ratio of the yoke so significantly that unacceptable scan non-linearity occurred.
It is an object of the present invention to provide a deflection yoke which permits the introduction of opposite direction current flow to that of a main winding at controlled locations about a deflection yoke and which does not decrease the L/R ratio of the yoke to unacceptable levels.